The present invention generally relates to washing machines, such as household or residential type washers, including both clothes washing machines and combined clothes washing and drying machines.
A washing machine, top-loading or front-loading, typically includes the following parts: an outer casing, with a structural base portion and a control panel; a washing assembly elastically connected with the outer casing (ie, hung on or supported by said casing), and including a tub and a substantially horizontal-axis rotating drum; a drive assembly with an electric motor adapted to operate the drum at a reduced rotational speed (for instance, 50-70 rpm) in the washing phases of the cycle, and at least one higher rotational speed (up to even 1500-1800 rpm in some models) during spin-extraction phases.
During operation, it may quite frequently occur that, in the washing machine, the items making up the washload distribute unevenly, ie. in a non-uniform manner, along the periphery of the rotating drum. As a result, the washing assembly of the machine is subject to stresses that tend to generate vibrations whose amplitudes increase with the rotational speed of the same drum and peak in the transient from washing to spin-extraction in correspondence of a rotational speed which is generally known as the "critical speed". In order to damp these vibrations, so as to prevent them from affecting the outer casing and being further transmitted on to the surrounding environment, in the form of noise or otherwise, the washing assembly is connected to the outer casing by means of springs and shock-absorbing struts.
In consideration of their simple, cost-effective construction, the shock absorbers that are most frequently used in this particular field of application are those of the friction type (see for instance EP-A-0 315 076 and EP-A-0 513 670) which, in view of ensuring a constant vibration-damping capacity thereof, are sized on the basis of standard operating conditions (extent of the stresses imparted, amplitude and frequency of the generated vibrations). These shock-absorbers, however, are "passive", failing to accommodate the fact that the actual operating conditions of the machine in use may differ considerably from standard conditions. In particular, such shock-absorbers have limited effectiveness in the presence of markedly anomalous oscillations, such as those generated by a washload with a reduced, but concentrated weight, as represented for example by a pair of tennis-shoes or a sponge-cloth towel or bathrobe. It should furthermore be noticed that, in these shock absorbers, a vibration damping action is actually performed even when it is not required (washing phases, perfectly balanced washload, etc.), and this may be the cause of a noise that is generally felt as unpleasant or disturbing by the users of the washing machine.
Washing machines are also known, for instance from EP-A-0 407 755, which make use of shock absorbers that are capable of producing a reduced vibration-damping effect in clothes washing phases of the cycle, as well as a second, higher vibration-damping effect in the spin-extraction phases of the process. All of these shock absorbers, considering their substantially "passive" character, have certain drawbacks. In particular, they tend to react with some delay, with an undesired damping peak, to the occurrence of sudden stresses when the amplitude of the oscillations of the washing assembly undergoes sharp variations.
Finally, EP-A-0 487 311 discloses a shock absorber for clothes washing machines that includes a hydraulic cylinder that is fixed to the washing assembly of the machine, and an actuator that is fixed to the outer casing of the washing machine and is triggered by an arrangement provided to control the rotational speed of the drum. A sliding drilled piston is enclosed in the cylinder, along with a fixed perforated partitioning baffle, and the operating rod of the piston protrudes from the cylinder. When the speed control arrangement detects a condition in which the drum is rotating at a low number of revolutions per minute, ie. a low speed, the actuator keeps a therewith associated retaining pin in a seat provided in the rod of the piston so as to damp the oscillations being transmitted by the washing assembly to the outer casing of the machine. When the speed control arrangement then detects a condition in which the rotational speed of the drum rises beyond a pre-determined threshold value, as occurs during the transition from the washing phase to the spin-extraction phase, the actuator retracts the retaining pin from said seat so as to positively interrupt any transmission of vibrations from the washing assembly to the outer casing of the machine. Such a shock absorber, however, apart from requiring close machining and manufacturing tolerances in connection with the retaining pin and the therewith associated seat in the piston rod, has limited reliability due to these two parts unavoidably undergoing considerable wear and tear due to repeated engaging and disengaging actions. Furthermore, the entire washing machine needs to be designed and constructed in an ad hoc manner due to the complexity of such a shock absorber as far as dimensions and construction are concerned.
It would therefore be desirable, and it is actually a main purpose of the present invention, to provide a method capable of effectively, reliably and inexpensively damping the vibrations at the very moment in which they are generated during the operation of the washing machine, even under non-standard conditions.
Another problem that generally needs to be overcome derives from the fact that, in the period of time between final testing at the end of the production line and the installation in the field, washing machines are unavoidably exposed to a number of external stresses, for instance shocks and jerks during transport. The need arises here in particular to prevent or damp bumping the washing assembly of the machines, which, is connected elastically with the outer casing. Now, all systems used by the various machine manufacturers to this particular purpose call for the installation, between the washing assembly and the outer casing of the machine, of appropriate locking means, such as threaded rods, bars, appropriately shaped or contoured shims of corrugated cardboard or expanded polystyrene, which must be removed before the machine is started. Such means are effective but, considering that they really have nothing to do with the actual operation of the machine and that their purpose is scarcely perceived by the consumers, they add in an undesired manner to the overall production costs of a washing machine. In any case, the presence of such locking means in a washing machine may prove quite inconvenient to the users, due not only to their need to be removed from the machine, but also to a substantial impossibility for those parts to be reused in some other different manner, as well as to a difficulty in disposing of these parts.